1. Field of the Invention
The present invention is directed to a magnetic resonance apparatus, and in particular to a magnetic resonance apparatus having vibration damping.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for, among other things, acquiring images of the inside of the body of an examination subject. In a magnetic resonance apparatus, rapidly switched gradient fields that are generated by a gradient system are superimposed on a static basic magnetic field that is generated by a basic field magnet. The magnetic resonance apparatus also includes a radio-frequency system that emits radio-frequency signals into the examination subject for triggering magnetic resonance signals and picks up the triggered magnetic resonance signals, from which magnetic resonance images are produced.
A magnetic resonance apparatus has an examination space in which a region of the examination subject to be imaged is positioned for producing magnetic resonance images of a selected region. To that end, the magnetic resonance apparatus usually comprises at least one support mechanism that is displaceable in a direction into and out of the examination space and on which the examination subject can be borne. By moving the support mechanism together with the examination subject seated thereon, positioning of the region to be imaged in the examination space is undertaken.
For example, U.S. Pat. No. 5,129,232 discloses a superconductive basic field magnet with a cryohead for a magnetic resonance apparatus. The operating cryohead is thereby known as a generator of mechanical oscillations that are transmitted onto the basic field magnet which have a negative influence on the uniformity (homogeneity) of the basic magnetic field as well as on the image resolution of the magnetic resonance apparatus. As a result undesired artifacts are produced in the magnetic resonance images. The vibrations generated by the cryohead typically have a dominant frequency of a few Hz.
Further, it is known that similar low-frequency mechanical oscillations having the above-described disadvantages can also be transmitted, for example, from the floor of the installation room of a magnetic resonance apparatus to the magnetic resonance apparatus. The aforementioned structure oscillations can be produced, for example, by neighboring street traffic and/or rail traffic and/or by other generators of mechanical oscillations arranged in the same building.
An object of the present invention is to provide an improved magnetic resonance apparatus with which, among other things, mechanical oscillations emanating from the environment wherein the magnetic resonance apparatus is installed can be rapidly and highly effectively damped in the direction of the magnetic resonance apparatus during the image pick-up mode of the magnetic resonance apparatus.
The above object is achieved in accordance with the principles of the present invention in a magnetic resonance apparatus having a controllable device with which at least a part of the magnetic resonance apparatus can be fixed in a first control state and with which the part can be held isolated, with respect to mechanical oscillations, in a second controlled state.
The invention is based on the perception that in a magnetic resonance apparatus having a damping mechanism for damping low-frequency mechanical oscillations between a part of the magnetic resonance apparatus and the environment wherein it is installed, low-frequency mechanical oscillations are also produced by a displacement of the support mechanism connected to the aforementioned part of the magnetic resonance apparatus. As a result, the damping mechanism during the actual generation of magnetic resonance images which immediately follows after the end of the displacement of the support mechanism, is still experiencing oscillations due to the displacement due to a comparatively long time constant of the damping mechanism for low-frequency oscillations. The full damping effect of the damping mechanism thus is not available for the actual magnetic resonance imaging. This situation for the displacement of the support mechanism also applies similarly to the procedure for positioning an examination subject on the support mechanism, removing the examination subject from the support mechanism as well as connection and disconnection of an active damping mechanism, for example in an embodiment as a controllable air cushion or a damping mechanism having piezo-electric elements.
The controllable device, with which the part of the magnetic resonance apparatus is fixed so as to be immobile relative to the surrounding structure during a time wherein the magnetic resonance apparatus is not operated with an imaging sequence or a spectroscopic sequence, therefore is arranged between this part of the magnetic resonance apparatus and a surrounding structure. As a result, a motion of the part relative to the surrounding structure is suppressed during this time. The aforementioned time duration particularly covers the above-described displacement of the support mechanism, the positioning and removal of the examination subject, the connection and disconnection of the active damping mechanism and further standstill times, for example standstill times at night, as well as times wherein maintenance of the magnetic resonance apparatus is implemented. In contrast, low-frequency mechanical oscillations emanating from the installation environment are highly effectively damped by the controllable device and, thus, disturbances of operations are prevented during imaging or stereoscopic sequence wherein the controllable device is in its second control status and has its full damping effect available.